1. Field of the Invention
This invention generally relates to submicron dimensional calibration standards and methods of manufacture and use. Certain embodiments relate to calibration standards having a traceably measured submicron lateral dimension that may be smaller than lateral dimensions of features which may be formed with current lithography equipment.
2. Description of the Related Art
As the dimensions of semiconductor devices continue to shrink with advances in semiconductor materials and fabrication processes, monitoring and controlling semiconductor fabrication processes by lateral dimensional metrology has become increasingly important in the successful fabrication of advanced semiconductor devices. Currently available systems which may be used for lateral dimensional metrology may include systems configured to perform a technique such as optical, electron beam, ion beam, atomic force, and scanning probe microscopy. In addition, lateral dimensional metrology systems may also include systems configured to perform an electrical metrology technique. For example, an electrical metrology technique may involve measuring resistance of a feature of a known material and determining a cross-sectional area and/or a linewidth of the feature from the measured resistance.
A calibration standard may be used to calibrate lateral dimensional metrology systems as described above. A calibration standard may include features such as lines and/or spaces having a certified lateral dimension. Currently available linewidth calibration standards may have a lateral dimension artifact of approximately 500 nm to approximately 30,000 nm. For example, such calibrations standards may typically be formed by semiconductor fabrication processes such as lithography and etch. Such lithography and etch processes may produce features having a lateral dimension of greater than approximately 180 nm. As such, a minimum lateral dimension of calibration standards formed by current lithography and etch processes may be limited by a performance capability of such processes and systems. In this manner, lateral dimensional metrology equipment may be calibrated at a minimum lateral dimension which may be substantially greater than a lateral dimension of features formed by advanced semiconductor fabrication processes. Lateral dimensional metrology equipment, therefore, may have limited usefulness for monitoring and controlling advanced semiconductor fabrication processes.
Several calibration methods for lateral dimensional metrology equipment, however, have been developed for use with currently available calibration standards to expand the usefulness of lateral dimensional metrology equipment for advanced processing applications. Examples of methods for expanding the usefulness of lateral dimensional metrology equipment for advanced semiconductor fabrication process applications are illustrated in U.S. Pat. Nos. 5,914,784 to Ausschnitt et al., 5,969,273 to Archie et al., and 6,128,089 to Ausschnitt et al., and are incorporated by reference as if fully set forth herein. Such methods, however, may include indirectly determining a location of an edge of a feature. Therefore, lateral dimensional calibration and measurement using such methods may be subject to substantial inaccuracy.
An example of a calibration standard may include conductive features formed on a insulating layer. For example, a silicon dioxide insulating layer may be formed below the surface of a monocrystalline silicon substrate using an implantation process which may be commonly referred to as xe2x80x9cSeparation by the Implantation of Oxygen,xe2x80x9d or xe2x80x9cSIMOX.xe2x80x9d The insulating layer may be annealed to form an amorphous layer of insulating silicon dioxide within the monocrystalline silicon substrate. The monocrystalline silicon layer above the insulating layer may have a defined crystal structure. In addition, the monocrystalline silicon layer above the insulating layer may be patterned using standard photolithography and etch techniques. In this manner, the monocrystalline silicon layer may be etched along a plane of the crystal structure to form silicon features having substantially planar sidewalls. Linewidth and line spacing of the silicon features may be measured using transmissive electron microscopy measurements and electrical measurements. Linewidth and line spacing of the silicon features may also be measured using xe2x80x9catomic lattice countingxe2x80x9d techniques such as scanning probe microscopy (xe2x80x9cSPMxe2x80x9d) because etching the monocrystalline layer along crystal planes may form a very accurate structure. Examples of such calibration standards are illustrated in U.S. Pat. Nos. 5,684,301 to Cresswell et al. and 5,920,067 to Cresswell et al., and are incorporated by reference as if fully set forth herein.
Such currently available calibration standards, however, may include opaque conductive features formed by standard lithography and etch techniques. In this manner, a lateral dimension of the opaque conductive features may be greater than or equal to a minimum lateral dimension of a feature that may be produced by currently available processes and systems. For example, opaque conductive features of such calibration standards may have a minimum lateral dimension of approximately 500 nm.
Another currently available calibration standard may include at least one pair of different structures such as a line and trench. Examples of such calibration standards are illustrated in U.S. Pat. Nos. 5,534,359 to Bartha et al., 5,665,905 to Bartha et al., and 5,960,255 to Bartha et al., and are incorporated by reference as if fully set forth herein. Such calibration standards may be used to calibrate an ultra-fine tip such as a tip which may be used for AFM or SPM. Calibration may include determining a width of the tip. A width of the tip may be determined by profiling a pair of different structures with the tip. For example, calibration of the tip may include measuring a width of a line by profiling the line with the tip. In addition, calibration of the tip may include measuring a width of a trench of the same pair of structures by profiling the trench with the tip. If the pair of structures have substantially equal lateral dimensions, then the measured widths of the line and the trench may be subtracted, and the resulting value may be divided by two to determine the exact diameter or width of the tip.
Once the exact diameter or width of the tip has been determined, the tip may be used to measure features and layers of additional samples. Therefore, it is very important that the calibration standard have at least one pair of different structures such as a line and a space which have exactly the same width. In addition, it is very important that two measurements are carried out with different structures of the same pair of structures to assure accurate calibration and subsequent accurate measurement. In this manner, knowledge of the exact dimensions of the features of the calibration standard is not necessary to calibrate the tip. Therefore, lateral dimensions of the features of the calibration standard may not be traceably measured. Traceable measurements may include measurements performed in a manner traceable to the National Institute of Standard and Technology (xe2x80x9cNISTxe2x80x9d). As such, the calibration standard may not be certified and may not be used to calibrate additional measurement systems such as optical microscopes, scanning electron microscopes, focused ion beam microscopes, and electrical metrology systems.
Accordingly, it would be advantageous to develop a calibration standard including at least one feature having a lateral dimension of less than approximately 500 nm which may be traceably measured, accurately certified, and relevant to a semiconductor fabrication process being monitored and controlled by a lateral dimensional metrology system calibrated with the calibration standard.
An embodiment of the invention relates to a calibration standard which may be used to calibrate lateral dimensional measurement systems. The lateral dimension of the standard may be produced and measured in a way traceable to NIST through the use of thin film deposition techniques and thin film metrology techniques. Lateral dimensional measurement systems may include, but may not be limited to, systems configured to perform a technique such as scatterometry and optical, electron beam, ion beam, atomic force, and scanning probe microscopy.
The calibration standard may include a first substrate spaced from a second substrate. In addition, the calibration standard may include at least one layer disposed between the first and second substrates. The layer may have a traceably measured thickness. For example, the traceably measured thickness may be determined by thin film metrology. The traceably measured thickness may also be determined using a traceable measurement technique such as ellipsometry, spectrophotometry, interferometry, profilometry, energy dispersive X-ray spectroscopy (xe2x80x9cEDSxe2x80x9d), thermal and acoustic wave techniques, cross-sectional transmission electron microscopy (xe2x80x9cTEMxe2x80x9d) and X-ray techniques. Each of the traceable measurement techniques described above may include calibration of a measurement system with a standard reference material traceable to a national testing authority such as NIST. Appropriate traceable measurement techniques may further include any measurement technique in which a measurement system may be calibrated with a standard reference material traceable to a national testing authority. Appropriate traceable measurement techniques may also include reference to the atomic spacing in monocrystalline silicon. The traceably measured thickness may be less than approximately 500 nm, or even less than approximately 100 nm.
The first substrate, at least the one layer, and the second substrate may be cross-sectioned in a direction substantially perpendicular to an upper surface of at least the first substrate. The calibration standard may also be rotated such that a cross-sectional surface formed by the cross-sectioned portion of the calibration standard may be a viewing surface of the calibration standard. In this manner, a lateral dimensional artifact of the calibration standard may include the traceably measured thickness of at least the one layer.
The viewing surface of the calibration standard may be planarized such that the viewing surface may be substantially planar. Alternatively, the viewing surface may be substantially non-planar. For example, a portion of the first and second substrates extending from the cross-sectional surface or a portion of at least the one layer extending from the cross-sectional surface may be removed such that the viewing surface may be substantially non-planar. In addition, a portion of at least the one layer extending from the cross-sectional surface may be removed to form a trench in the calibration standard between the first and second substrates. Furthermore, a thermally grown oxide may be formed on silicon surfaces of the calibration standard such as upper surfaces of the first and second substrates and sidewall surfaces of the trench. Subsequent processing may include removing the thermally grown oxide. As such, reentrant knife-edge structures may be formed at upper corners of the trench. Alternatively, a portion of the first and second substrates extending from the cross-sectional surface may be removed such that at least the one layer may form a topographic feature of the calibration standard. In this manner, the topographic feature may be a line or another appropriate topographic feature.
In an embodiment, materials of the calibration standard may include materials used in a semiconductor fabrication process. Furthermore, a traceably measured thickness of at least one layer of the calibration standard may be approximately equal to a lateral dimension of a feature formed by the semiconductor fabrication process. The lateral dimension of a feature formed by the semiconductor fabrication process may be measured with a lateral dimensional measurement system calibrated with the calibration standard. As such, the materials of the calibration standard and the traceably measured thickness of at least the one layer may be relevant to a feature formed by the semiconductor fabrication process. In this manner, a calibration standard may be relevant to a semiconductor fabrication process that may be monitored and/or controlled by lateral dimensional metrology.
In an embodiment, a calibration standard may include at least three layers disposed between the first and second substrates. At least the three layers may form at least one lateral dimensional artifact, or feature, of a calibration standard. For example, the three layers may form a plurality of features such as two lines, two trenches, and a line and a space. In addition, the three layers may include a first feature and a second feature. A traceably measured thickness of the first feature may be approximately equal to a traceably measured thickness of the second feature. Alternatively, a traceably measured thickness of the first feature may be substantially different than a traceably measured thickness of the second feature. Furthermore, at least the three layers may include at least two features of a repetitive pitch grating. In this manner, the calibration standard may include any number of lateral dimensional artifacts or features.
An additional embodiment relates to a method for forming a calibration standard. The method may include forming at least one layer upon an upper surface of a first substrate. The method may also include determining a thickness of at least the one layer using a traceable measurement technique. Determining a thickness of at least the one layer may include calibrating a measurement system with a standard reference material traceable to a national testing authority. For example, the traceable measurement technique may include thin film metrology. The traceable measurement technique may also include, but may not be limited to, ellipsometry, spectrophotometry, interferometry, profilometry, EDS, thermal and acoustic wave techniques, cross-sectional TEM, and X-ray techniques. Each of the traceable measurement techniques described above may include calibrating a measurement system with a standard reference material traceable to a national testing authority such as NIST. The determined thickness may be less than approximately 500 nm, or even less than approximately 100 nm.
The method may include bonding a second substrate to an upper surface of at least the one layer. The method may further include cross-sectioning the first substrate, at least the one layer, and the second substrate in a plan substantially perpendicular to at least the first substrate. In this manner, cross-sectioning may form a viewing surface of the calibration standard such that a lateral dimensional artifact of the calibration standard may include the determined thickness of at least the one layer. The method may also include planarizing the cross-sectioned first substrate, at least the one layer, and second substrate such that the viewing surface may be substantially planar. In addition, the method may include removing a portion of the first substrate, at least the one layer, and the second substrate extending from the viewing surface such that the viewing surface may be substantially non-planar.
The method may also include removing a portion of the calibration standard to form at least one topographic structure of the calibration standard. For example, the method may include removing a portion of the first and second substrates extending from the viewing surface such that at least the one layer may form a topographic feature of the calibration standard such as a line. Alternatively, the method may include removing a portion of at least the one layer extending from the viewing surface to form a trench or a space in the calibrations standard. In addition, or as an alternative, to traceably measuring a thickness of at least the one layer subsequent to deposition of the layer, a thickness of the layer may be measured with a traceable measurement technique subsequent to cross-sectioning and additional processing as described herein.
In an embodiment, the method may include forming a thermally grown oxide on upper surfaces of the first and second substrates and on sidewall surfaces of a trench formed in the calibration standard. The method may include removing the thermally grown oxide to form reentrant knife-edge structures at upper comers of the trench. Alternatively, the method may include depositing a material on the thermally grown oxide. The method may also include planarizing the deposited material and the thermally grown oxide such that upper surfaces of the thermally grown oxide and the deposited material are substantially planar with upper surfaces of the first and second substrates. The method may further include determining a thickness of at least the thermally grown oxide or the deposited material using a traceable measurement technique. Furthermore, the method may include removing a portion of the thermally grown oxide and/or the deposited material to form topographic features such as trenches in the calibration standard.
An additional embodiment relates to a method for calibrating a lateral dimensional measurement system. The method may include determining a thickness of at least one layer of a calibration standard with the lateral dimensional measurement system. The calibration standard may be configured as described herein. For example, at least one layer of the calibration standard may have a traceably measured thickness. In addition, a lateral dimensional artifact of the calibration standard may include the traceably measured thickness of at least the one layer. The method may also include altering calibration factors of the measurement system if the determined lateral dimension is not substantially equal to the lateral dimensional artifact of the calibration standard.
In an embodiment, a calibration standard may include features such as lines and/or spaces. A calibration standard may also include pitch features with repetitive lines since lateral magnification calibration of a microscope may be accomplished with lower errors from line edge determination than from width measurements. Such features may have accurately characterized sidewall angles and precisely measured lateral and/or vertical dimensions. Lateral dimensions of features of the calibration standard may be measured in a manner traceable to the National Institute of Standards and Technology (xe2x80x9cNISTxe2x80x9d) or any other competent standard authority. In addition, such features may have lateral and/or vertical dimensions which may be approximately equal to lateral and/or vertical dimensions of features formed by a semiconductor fabrication process. In this manner, features of a calibration standard may have lateral dimensions of less than approximately 250 nm. For example, features of a calibration standard may have lateral dimensions of approximately 100 nm or less. As such, lateral dimensions of a calibration standard may be relevant to lateral dimensions of features formed by processes being monitored or controlled by lateral dimensional metrology. Furthermore, a lateral dimensional artifact of the calibration standard may be smaller than a lateral dimension of a feature that may be formed by currently available lithography equipment.